1. Field of the Invention
The present invention relates to a digital communication system comprising a transmitter for transmitting digital messages representing symbols modulated onto at least one carder, which symbols are representable as symbol vectors in a signal constellation which is rotationally invariant over a predetermined rotation angle, the system further comprising a transmission channel and a receiver for receiving the symbols transmitted through the channel, which receiver comprises a demodulator for demodulating the received symbols into received messages, the demodulator using an estimate for a channel state of the channel. Such a system can be a Digital Television Broadcasting System in which a digital signal is broadcasted to digital TV receivers, or any other digital communication system.
The present invention further relates to a receiver for use in such a system.
2. Description of the Related Art
A system of this kind is known from U.S. Pat. No. 5,345,440 and the International patent application WO 92/05646. In this system OFDM (Orthogonal Frequency Division Multiplexed) signals are transmitted by a transmitter to a receiver through a transmission channel. On each carrier of this multi-carrier system, symbols to be transmitted are QPSK (Quadrature Phase Shift Keying) modulated. An OFDM signal is obtained by supplying a QPSK symbol stream to an Inverse Discrete Fourier Transformer. The QPSK signals can be considered as two bit binary messages representing symbols on a carrier. The symbols can be represented as symbol vectors in a signal constellation which is rotationally invariant over a predetermined angle. In the example given the signal configuration consists of four constellation points, representing two bit binary messages "00", "10", "11", and "10". In case of differential encoding, starting from an initial symbol state, the messages are encoded as a phase difference between two successive symbols. Then, in the receiver, it is not necessary to know the exact phase, but, starting from a first detected symbol, the succeeding symbols are detected from relative phases of symbols. The transmitted symbols are subject to noise and to channel distortion. Therefore, the received symbols are amplitude and phase shift modulated QPSK signals. In a complex plane, the received symbols are scattered around four points of a four point signal constellation, or, stated otherwise, when many symbols are received the received symbols can be plotted as four clouds of end points of received symbol vectors in the complex plane. Before proper demodulation, a position of two notional perpendicular axes, which are symmetrical to center of the signal clouds, must be found. For this reason, in the known receiver, a separate phase recovery method is disclosed for each subcarrier of the received OFDM signal. In this method, with demodulation, a channel state is estimated, which varies with time. More particularly a channel phase shift is estimated recursively, i.e. with each of the received symbols, the position of the notional axes with respect to the symbol clouds is updated. Starting with a phase reference signal which is set equal to the first received signal, a reference phase which is set equal to an argument of the phase reference signal, and notional axes at 45.degree. and 135.degree. to the signal vector, a next received symbol is rotated with a multiple of 90.degree. such that it falls within 45.degree. sectors at both sides of the reference phase. That is, a minimum squared Euclidean distance is determined, when detecting the symbol. From the number of 90.degree. rotations, the transmitted binary message can be detected. The phase reference signal is updated with the received rotated vector, and, consequently, the reference phase. This method is shown to be Maximum Likelihood in most practical cases. In digital communication systems, also symbols represent constellations are transmitted. For example for Digital Video Broadcasting a so-called 2R(2,2)-QAM constellation can be used. Such a constellation is a 2-resolution configuration, i.e. a constellation representing symbols of which a first group of 2 bits for encoding the symbols represents constellation points in respective quadrants of the signal constellation, and of which a second group of 2 bits for encoding the symbols represents constellation points within the quadrants. The first group of bits are so-called high priority bits, and the second group of bits are so-called low priority bits. When symbols representing video signals are transmitted using such a 2R(2,2)-QAM signal constellation, at the receiver side, depending on the signal-to-noise ratio, the video signals can be detected with low resolution by only detecting the high priority bits and the video signals can be detected with high resolution by also detecting the low priority bits. The 2R(2,2)-QAM constellation is rotationally invariant as to rotation of clusters of constellation points in the respective quadrants over a multiple of 90.degree.. In this sense, also other signal constellations, which can be multiresolution or not, are rotationally invariant, such as a 64-QAM constellation. Such signal constellations is described in the handbook "Digital Communication", E. A. Lee et al., Kluwer Academic Publishers, 1990, Chapter 6.5. "Signal Set Design", pages 198-206. In the same handbook transmitters and receivers are described for transmitting and receiving of symbols from such constellations.